STEAM TURBINE

The steam turbine is a turbine in which the potential energy of heated and compressed steam produced in a special device, a steam generator, or steam of natural origin (for example, from geothermal springs) is converted into kinetic energy (when the steam expands in the turbine blade cascades) and then into mechanical work on the rotating shaft. The rows of rotating blades fixed on the steam turbine rotor change the total steam enthalpy and positive work is done. In the gas turbine (see Gas Turbine) the pressure ratio πT (that is the ratio of the working fluid pressure at the turbine inlet to the pressure at the turbine outlet) is not very large (usually not higher than 20-30) but the initial temperature of the gas (combustion products) may be as high as 1700-1800 K. In contrast, the steam turbine is characterized by larger pressure ratios πT ≈ 2000-6000 (due to higher initial values (ph) and low final values (pt) of the steam pressure) and considerably lower initial steam temperature (Th ≈ 810-880 K). Therefore, the enthalpy drop in the steam turbine is 2-3 times higher than that in the gas turbine, and the number of stages in the steam turbine is many times larger than that in the gas turbine.

There are several types of steam turbines shown schematically in Figure 1. In a condensing turbine (Figure 1a) the steam is expanded down to the deep vacuum (pt ≈ 4-3 kPa) reached in the condenser. These turbines are designed with uncontrolled steam bleed used for feed water regenerative heating. The uncontrolled bleed is characterized by unsteady pressure of the extracted steam. The steam is bled through a special manifold in the bottom part of the turbine casing.

Figure 1b is a schematic of a turbine with condensation and with one controlled steam bleed for process and domestic heat demands. In these turbines, a portion of steam is bled from intermediate stages to be used by consumers. The remaining portion of the steam passes the subsequent turbine stages and after that passes to the condenser. The bleed pressure is kept steady regardless of the turbine load, a special regulator device being used for this purpose. In the turbine shown in Figure 1c, there are two controlled steam extractions at different pressures.

Figure 1d is the schematic of a turbine with two pressures. This turbine uses not only fresh steam from the boiler, but also exhaust steam from hammers, presses, pump, air blower and compressor drives.

A backpressure turbine is shown in Figure 1e. There is no condenser in such a turbine unit. The steam at the required pressure is fed from the turbine and used for processes and for domesic needs.

Steam turbine design is influenced by the turbine capacity, initial steam parameters (sub- and supercritical), its operation conditions within the power generation system (base-load, peak-load, semi-peak load), final steam moisture content, technological characteristics and other factors. Low capacity turbines (up to 50 mW) are as a rule of one-cylinder type.

The disadvantages of high capacity condensing turbines are connected with the limited flow rates of the final stages. To overcome this difficulty, these turbines are constructed with division of the main steam flow (before it enters the final stages) into several parallel flows. Each part of these turbines is designed for the maximum steam flow rate Qm (Figure 2.)

These parts are referred to as a high pressure cylinder, an intermediate pressure cylinder and a low pressure cylinder.

Steam turbine rotors may be disk-type or drum-type (Figure 3), Disk-type rotors are used in impulse turbines (see Turbines), dram-type rotors are used in reaction turbines.

Steam turbines are used as parts of stationary and transport (marine) steam turbine power units. Besides turbines these power units also include boilers (steam-generators), steam condensers and other devices. Steam turbines constructed for combined operation with gas turbine units are also used as parts of combined steam-gas plants (see Gas Turbines) with applications in both stationary and transport (marine) power units.

The real cycle apbb'hta (Figure 4) of the simplest steam turbine unit includes the ap process of increasing pressure of the water in the pump, the pb process of heating water at constant pressure to the boiling temperature, and bb' process of evaporation at constant temperature. The b'h process corresponds to water superheating, the ht process corresponds to the expansion of steam in the turbine. The ta process which is the closing process of the cycle corresponds to heat removal in the condenser.

Figure 1. Types of steam turbine.

Figure 2. Multistage steam turbine.

Figure 3. Disk (top) and drum (bottom) types of rotor design.

Figure 4. Steam turbine cycle on temperature entropy (T-S) diagram.

This real cycle of the steam-turbine unit differs from the ideal thermodynamic cycle ap'bb'h't'a because of irreversible losses in the pump, steam pipe, turbine and condenser. These losses are denoted by I, II, III and IV areas (Tx is the temperature of water used for cooling the condensate). The specific work of the real cycle le = lt — lp, where lp = ih — it is the actual turbine work (where ih and it are the steam enthalpy at the beginning and at the end of the expansion process in the turbine); lp = ip — ia is the specific work of the pump in the real cycle (where ip and ia are the enthalpy of water at the corresponding points of the cycle). The ideal cycle thermal efficiency is ηt = (h' — it')/(h' — ia) ; the effective efficiency of the steam-turbine unit is ηe = ηtηTηm , where ηT and ηm are the efficiency of the turbine itself and the efficiency taking into account the mechanical losses in the turbine. When determining the efficiency ηt of a steam turbine it is necessary to take into account the moisture of the steam which is typical for the last stages of steam condensing turbines and for many stages of turbines using saturated and slightly superheated steam (these turbines are used, for instance, at nuclear power stations). When such steam is used the efficiency of the stages decreases. In this case relative losses ζw may be rather large (for example, in the last 3 stages of a turbine of 800 mW capacity and with initial steam pressure 24 MPa ζw = 0.012 to 0.081; still greater losses due to moisture are typical for turbines without intermediate heating). Besides that the first stages of steam turbines are often with partial admission (ε ≈ 0.15), and ventilation losses occur in them (see Turbine). In intermediate stages of heavy-duty steam-turbines using superheated steam, the maximum blading efficiency ηb = 0.905 to 0.903.

Reaction ratio at the middle diameter in the high pressure and intermediate pressure cylinders of steam turbines increases with the number of stages from 0.2 to 0.4, and in the low pressure cylinder from 0.3 to 0.7.

A variety of techniques are used for increasing the efficiency of steam-turbine units. One of the methods is increasing initial parameters of the steam. For example, when pressure ph is increased, the saturation temperature increases. The result is an increase of the average temperature at which heat is supplied; thus the thermal efficiency ηt of the ideal cycle increases too. However, in practice an increase of pressure to the value more than 9-10 MPa does not result in the increase of the theoretical work and does not significantly affect the unit efficiency. Also, steam moisture content at the end of the expansion process increases with the increase of pressure and results in greater losses in the course of the steam expansion and also in the turbine blade erosion. Therefore, the general tendency is to limit the moisture content to 13-15 per cent.

Simultaneous increase of the values of ph and Th may considerably increase the steam-turbine unit efficiency. For this purpose many present-day steam-turbine units have intermediate (repeated) superheating of the steam after expansion in the first group of stages. In this case the theoretical work of the turbine, the cycle work and thus the cycle effiency increase, the moisture of the steam at the end of the expansion process decreases, and the amount of heat transfered in the condenser increases. The temperature of superheating as well as the initial temperature is limited by the thermal characteristics of the flow passage metal parts.

A decrease of the steam pressure pt in the condenser causes a decrease of the steam condensation temperature and consequently increases the temperature difference in the cycle.

The efficiency of steam-turbine units increases when regenerative extraction of steam from the turbine is used. Regenerative extraction is uncontrolled bleed of the steam from the stages with the aim of increasing the feed water temperature in the unit. In this cycle the feed water is heated by the heat released in the process of the steam cooling and condensation.

Condensing steam turbines have an efficiency in the range ηe = 36 to 42%. From this it follows that only a small portion of heat released in the process of fuel combustion is transformed into effective work. Turbine units for power and steam generation have higher overall efficiency. In these units the heat from the fuel is used for power generation and for obtaining heat at some prescribed temperature level. .The theoretical work of the unit with the steam turbine for power and heat generation is less than that of the steam turbine unit with condensing steam turbine. The useful work of the cycle of the steam turbine unit for power and heat generation is also lower than that of the condensing turbine. However, the steam turbine unit for combined power and heat generation makes effective use of the heat of condensation and therefore its overall efficiency is higher than that of a condensing steam turbine unit.